Elastomeric films have for some time been used and discussed in the literature with regard to their applications in disposable products, such as baby diapers and adult incontinent devices. These elastomeric webs or films are used primarily in the body hugging portions of garments. In diapers, for example, elastomeric bands are typically used in the waistband portions such as discussed in U.S. Pat. No. 4,681,580, issued to Reising et al., and Lash, U.S. Pat. No. 4,710,189. Both these patents describe the use of elastomeric materials which have a heat stable and a heat unstable form. The heat unstable form is created by stretching the material when heated around its crystalline or second phase transition temperature followed by a rapid quenching to freeze in the heat unstable extended form. The heat unstable elastomeric film can then be applied to the, e.g., diaper and then heated to its heat stable elastomeric form. This will then result in a desirable shirring or gathering of the waistband of the diaper. A problem with these materials, other than cost, is the fact that the temperature at which the material must be heated to release the heat unstable form is an inherent and essentially unalterable property of the material to be used. This extreme inflexibility can cause severe problems. First, it is more difficult to engineer the other materials with which the waistband is associated so that they are compatible with the temperature to which the elastomeric member must be heated in order to release the heat unstable form. Frequently this temperature is rather high which can potentially cause significant problems with the adhesive used to attach the elastomeric waistband, or, e.g., the protective back sheet or top sheet of the diaper. Further, once chosen the elastomer choice can constrain the manufacturing process rendering it inflexible to lot variations, market availability and costs of raw materials (particularly elastomer(s)), customer demands, etc.
Elastomers discussed in the above two patents, suitable for use in diapers, include those described in more detail by Massengale et al., U.S. Pat. Nos. 3,819,401, Koch et al., 3,912,565, Cook U.S. Pat. RE 28,688 and commercial materials, which are believed to correspond to those described in Hodgson et al., U.S. Pat. No. 4,820,590 issued to Exxon Chemical Patents Inc. Massengale et al. describes an elastomeric member which can be used to shirr flexible articles such as diapers, in the waistband and leg portions. The polymers described are polyvinyl chlorides containing one or more specific plasticizers. The polymers specifically described required heating to a temperature of 100.degree. C. in order to cause the tape to shrink to its heat stable elastomeric form. The polyvinyl chloride heat shrinkable elastomer was a proposed solution to the problems associated with attaching a conventional rubber or elastic material to a shirred article, which was required to be held in a stretched condition for it to be attached to the shirrable sheet or an even more problematic attachment to the sheet in the shirred condition.
Koch et al. describes a polyurethane heat shrinkable elastomeric material which can be used to shirr the waistband and leg portions of a diaper. The polyurethane was described as conventional heat shrinkable polyurethanes and again had to be heated to 100.degree. C. to release the elastomeric tape to its heat stable contracted form.
An allegedly novel composition was proposed by Hodgson et al. whose elastomer is a blend of three components including predominately an ethylene copolymer elastomer modified by olefinic elastomer and process oil. A heat unstable form was again created by heating and stretching the materials, at a temperature of preferably not more than 10.degree. F. below the crystalline melting point of the ethylene copolymer components of the composition. The material was then cooled, freezing in the heat unstable form. The stretched unstable material was then activated or relaxed at approximately the temperature at which the film was stretched. Although this material resumed its heat stable form at a lower temperature than the materials described in the above two patents, this temperature is still determined at the molecular level and is not subject to significant modification once the elastomeric material of choice is selected.
Other materials and methods have been proposed, for example Berger, U.S. Pat. No. 3,694,815, proposed a method for attaching a stretched relaxed elastic ribbon to a garment by stretching conventional elastic ribbons and immediately freezing the elastomeric material at relatively extreme low temperatures (e.g., well below ambient). This process would obviously severely constrain the processing conditions and materials which could be used when attaching the elastomeric strand to its backing. UK Pat. Application 2190406 A proposed maintaining a conventional elastomer in a stretched condition, while attaching to the member to be shirred (e.g., a diaper), by a rigidifying member, which would then be removed or destroyed following the attachment procedure. As described, the elastomers are first stretched then applied to the rigidifying member in its stretched form. Finally, Meltray et al., UK Pat. 2,160,473 proposes an elastomer which will shrink at an elevated temperature (e.g. at or above 175.degree. F. or 79.4.degree. C.). The allegedly novel feature of this material, compared to the heat shrink materials discussed above, is that it does not require preheating during the stretching operation but rather could be stretched at ambient temperatures by a differential speed roll process or by "cold rolling". The polymer proposed was a copolymer having alternating segments of polyamidepolyether block polymers, commercially available under the trade name Pebax, particularly Pebax Extrusion grades 2533 and 3533. As an alternative this patent application proposed placing a thin EVA(ethylene-vinyl acetate) layer(s) over the elastomer by, e.g., coextrusion. The skin layer is chosen to prevent blocking or to be compatible with a later applied adhesive. It was noted that this layer can also produce a pleasing hand but so as not to interfere with heat shrinkability.
Problems with these elastomeric films include the difficulties inherent in applying a stretched elastic member to a flexible substrate such as a disposable diaper. Although some of the elastomers proposed have the advantage that they can be applied at ambient conditions in a heat stretched unstable form, subsequent often extreme heating is required to release the heat unstable form to a contracted heat stable form. The temperature of this heat release is generally inflexible as it is determined at the molecular level of the elastomer. As such the other materials applied to the elastomer, and the process conditions at which the elastomer is used, must be carefully selected to be compatible with this heating step.
Elastomers also exhibit relatively inflexible stress/strain characteristics which cannot be chosen independently of the activation temperature. Materials with a high modulus of elasticity are uncomfortable for the wearer. Problems with a relatively stiff or high modulus of elasticity material can be exaggerated by the coefficient of friction and necking of the elastomer which can cause the material to bite or grab the wearer.